Dynamic foil display having low resistivity electrodes

ABSTRACT

A dynamic foil display  900  is provided wherein the spacer elements  902, 904  also serve as resistance-reducing tracks for the electrode circuitry. Resistance-reduction can thereby be provided for the row and column electrodes as well as for the foil electrode. To this end a foil display  900  comprises a light guide plate  905 ; a passive plate  910 ; and a transparent light-scattering foil  903  sandwiched between and separated from said plates by means of spacer elements  902, 904  is described. The spacer elements  902, 904  are arranged essentially along rows and columns on said plates  905, 910  and thereby define a plurality of pixel elements arranged in a matrix configuration along said rows and columns. The display further comprises an electrode circuitry which comprises a transparent foil electrode  907  arranged laterally on said foil and transparent row and column electrodes  901, 911  arranged along said rows and columns on said light guide and passive plates; and the spacer elements  902, 904  are electrically conducting and interconnected with said electrodes  907, 911, 901  and thus form part of said electrode circuitry.

A Dynamic Foil Display (DFD) typically comprises a display panel havingan active light guide plate, a passive plate, and a movable,light-scattering foil sandwiched between these plates. The foil isarranged with a transparent common foil electrode substantially coveringthe surface area of at least one side of the foil. Pixels are typicallyarranged in a matrix configuration, each pixel being located at theintersection of a horizontal, transparent row electrode arranged on thepassive plate and a vertical, transparent column electrode arranged onthe active plate. Each pixel is separated from its neighbors by means ofspacers, deposited between and spatially separating the foil from theactive and passive plates. The spacers are typically 1 micrometer inheight, and, at least on the light guide plate, are formed out of aspecular-reflective material possessing a high reflectivity towardslight in the visible wavelength region.

Depending on the voltage setup between the row, column, and foilelectrodes, electrostatic forces can be created locally forcing the foilinto contact with either the active light guide plate or the passiveplate, resulting in the pixel being either activated (light-emitting) orinactivated (dark), respectively. In order to avoid cross talk betweenneighboring pixels due to unintended foil deformations, the spacers aretypically arranged along rows and columns so as to surround each pixel.In a sense, each pixel thus constitutes a separate pixel cell. To avoidelectrical short-circuiting between the foil electrode and the plateelectrodes, the spacers are arranged on the display plates at locationsin between adjacent plate electrodes and are thus kept insulated fromthem.

The light guide is coupled to a light source, which typically isedge-mounted on the light guide. In case a pixel is activated, themovable foil is locally brought into contact with the light guide plateand light is decoupled out of the light guide plate into the foil whereit is scattered out of the display, resulting in a bright,light-emitting pixel. The pixel remains in this active state until it isdeactivated (darkened), i.e. until the foil-light guide contact isinterrupted, and vice versa.

Arranging a suitable color filter onto the passive plate portion of eachpixel provides for color pixels or sub-pixels. In order to provide forRGB displays, the pixels are typically arranged in sub-pixel groups,each sub-pixel group constituting a RGB pixel and consisting of a red, agreen and a blue sub-pixel.

According to current designs, the row and column electrodes and the foilelectrode are all constituted by transparent ITO (Indium Tin Oxide)layers having limited thickness in order to minimize light losses and/orcolor shifts due to incurred light absorption in the ITO electrodes. TheITO thickness is typically as small as 30 nm. However, having such thinelectrodes results in a limited electrical conductivity. In fact, theRC-time (Resistance-Capacitance time, the time needed for addressing) ofthe row and column electrodes in the foil display environment, which ischaracterized by a relatively high pixel capacitance C, is so long thatvoltage pulses imposed on these electrodes by voltage drivers duringdisplay operation cannot be sent with a sufficiently high speed acrossthe entire length of the electrodes.

To reduce the electrode RC-time, a metallic resistance-reducing track isdeposited on top of parts of the ITO row and column electrodes. Theseresistance-reducing tracks typically consist of a 100-200 nm thick and20-25 micrometers wide aluminum strip located on and along one or bothedges of the respective ITO electrodes. The advantage of this design isthat the RC time of the ITO electrodes is reduced (indeed due to thepresence of the resistance-reducing metallic tracks) while the risk forspacer-induced electrical short-circuit formation between the foilelectrode and the row/column electrodes is kept minimal since thespacers are still electrically insulated from the row and columnelectrodes and also from the metallic resistance-reducing tracksdisposed on the row and column electrodes.

Electrical short-circuit formation between the column electrodes and thefoil electrode and between the row electrodes and the foil electrode istypically prevented by means of an insulating layer covering the row andcolumn electrodes.

However, drawbacks of the current designs include:

The width of the metallic resistance-reducing tracks on the ITO row- andcolumn electrodes cannot be made too large, otherwise a significantpixel aperture reduction occurs, while the height of theresistance-reducing tracks must be kept below about 200-300 nm,otherwise their protruding presence interferes with the switchingbehavior of the foil between the active plate and the passive plate andvice-versa. These constraints result in an insufficient decrease of theRC-time even when the ITO row and column electrodes are provided withresistance-reducing tracks, in particular when large-area displays areconcerned.

It has been experienced that when resistance-reducing aluminum tracks offinite width and height are deposited directly on top of ITO (for thepurpose of making ohmic contact with the ITO), a chemical (probablyelectro-chemical) reaction occurs between the aluminum and the ITOleading to a darkening of the aluminum/ITO interface. On the light guideplate in particular, this induces unwanted light absorption and thus aloss of light and possibly also a color shift. Furthermore, the adhesionbetween aluminum and ITO appears to be weaker than the adhesion betweenglass (SiO₂) and Al, resulting in a ready delamination of the aluminumresistance-reducing tracks from the ITO electrodes during processing.Aluminum can indeed be replaced by silver, but silver has the drawbackof a poor chemical stability over time leading to a darkening effect andthus also unwanted loss of light. Replacing aluminum or silver withanother metal (such as chromium or titanium), leads to a significantlyincreased absorption of light and is therefore not a good solution.

A large number of process steps are required to manufacture/structurethe ITO electrodes, the resistance-reducing tracks, and the spacerpatterns on the panel plates.

The resistance-reducing tracks do not improve the conductivity of thefoil electrode, which of course also affects the RC-time and thus theaddressing performance of the display.

An object of the present invention is thus to provide improved foildisplays, in which the implications of the above drawbacks arealleviated. This object is achieved in the foil display as defined inthe appended independent claim, and the appended sub-claims providepreferred embodiments of the invention.

Whatever the layout, a common characteristic of prior art designs isthat the spacers are not part of the row/column/foil electrodecircuitry. For the purpose of the present invention, it has beenrealized that a spacer element and a resistance-reducing trackadvantageously can be incorporated into one single element.

Thus, according to one aspect of the invention a foil display isprovided comprising: a light guide plate; a passive plate; and atransparent light-scattering foil sandwiched between and separated fromsaid plates by means of spacer elements. The spacer elements arearranged essentially along rows and columns on said plates and therebydefine a plurality of pixel elements arranged in a matrix configurationalong said rows and columns. The inventive display further comprises anelectrode circuitry comprising a transparent foil electrode arrangedlaterally on said foil and transparent row and column electrodesarranged along said rows and columns on said light guide and passiveplates. The spacer elements are electrically conducting andinterconnected with said electrodes and thus form part of said electrodecircuitry.

From one point of view, the invention is thus based on the insight thatthe resistance-reducing tracks can be moved to a large degree aside fromthe ITO electrode layers and instead be mainly deposited directly on theglass plate, in parallel with the ITO electrode layers. Sufficientelectrical contact between a resistance-reducing track and theassociated ITO layer can be provided by means of limited-area electricalcontact points distributed along the ITO electrode layer. This isadvantageous in that the risk for delamination of theresistance-reducing track from the ITO electrodes is reduced, since theadhesion of aluminum on glass is much better than it is on ITO. Thereby,also, the light absorbing areas (which become darkened due to saidchemical reactions between the aluminum and the ITO) are substantiallyrestricted in size to the areas of the contact points only, instead ofbeing spread all along the entire plate areas occupied by theresistance-reducing tracks.

From another point of view, the invention is based on the insight thatspacer elements can form part of the electrode circuitry, and thuseliminate the need for additional resistance-reducing tracks on thetransparent electrodes.

According to one embodiment, some of the spacer elements are essentiallyparallel with said row electrodes and electrically interconnectedtherewith. For example, in cases where row directed electrodes arearranged on the passive plate these can be electrically incorporatedwith the respective electrodes and thus be made to substitute theresistance-reducing tracks associated with the ITO row electrodes. Thiscan be achieved by forming the row-directed spacers at least partly froma conductive material, by establishing electrical contact between therow-directed spacers and their associated ITO row electrodes by means oflimited-area contact points distributed along the ITO row electrodelayers, and by proper electrical insulation of the conductive part ofthe row-directed spacers from the foil electrode.

According to one embodiment, some of the spacer elements are essentiallyparallel with said column electrodes and electrically interconnectedtherewith. For example, in cases where column directed electrodes arearranged on the light guide plate these can be electrically incorporatedwith the respective electrodes and thus be made to substitute theresistance-reducing tracks associated with the ITO column electrodes.This can be achieved by forming the spacers at least partly from aconductive material, by establishing electrical contact between thespacers and their associated ITO electrodes by means of limited-areacontact points distributed along the ITO column electrode layers, and byproper electrical insulation of the conductive part of the spacers fromthe foil electrode. When the foil electrode is arranged on the foil sidefacing the passive plate, electrical insulation between the foilelectrode and the column electrodes including the spacers is at leastpartly provided for by the foil material itself, since this typically ismade out of an insulating material.

Obviously, the row electrodes can instead be arranged on the light guideplate and the column electrodes are thus arranged on the passive plate.In such cases, the above embodiments are readily applicable simply byapplying the description for the passive plate to the light guide plateand vice versa.

For both the column electrodes and the row electrodes, proper electricalinsulation from the foil electrode can be provided for by means of aninsulating layer deposited onto the ITO part of the plate electrodes andon the electrically conducting part of the spacer elements associatedwith the plate electrodes, thus effectively insulating the spacersassociated with the plate electrodes and the ITO of the plate electrodesfrom the foil electrode.

According to one embodiment, some of the spacer elements on at least oneof said plates are electrically interconnected with said foil electrodeand are spatially separated from and extends in a direction essentiallytransversal to the remaining spacer elements on said at least one plate.For example, in case the foil electrode is arranged on the passive plateside of the foil and the row directed electrodes are arranged on thepassive plate, column-directed spacer elements on the passive plate canbe electrically incorporated with the foil electrode and thus be made toserve as resistance-reducing tracks for the foil electrode. This isadvantageous in that the resistance of the foil electrode issubstantially reduced. It can be achieved by forming the column-directedspacer elements on the passive plate at least partly from a conductivematerial on the side of column-directed spacer elements facing the foil,by arranging the foil electrode at least on the foil side facing thepassive plate, by establishing electrical contact between the foilelectrode and the column-directed spacer elements, and by electricallyinsulating the column-directed spacer elements from the row electrodesand the row-directed spacer elements that are electrically incorporatedwith the row electrodes. Proper electrical contact between the foilelectrode on the foil side facing the passive plate and the columndirected spacer elements is naturally provided for as a result of thesandwiching of the foil between the spacer elements arranged on thelight guide plate and the passive plate, respectively. In case the foilelectrode is arranged on the light guide side of the foil, some of thespacer elements on the light guide are instead in similar fashioninterconnected with the foil electrode.

According to one embodiment, the transparent electrodes are formed outof ITO and the spacer elements optionally comprise aluminum.

According to another embodiment, the spacer elements which areelectrically interconnected with the row or column electrodes aredeposited partially on the respective transparent electrodes andpartially on the respective plate. Thus, the light absorbing areas canbe restricted in size while sufficient electrical contact is stillprovided for between the spacer elements and the associated ITOelectrodes on the active and passive plates, respectively. Preferably,the contact between the ITO electrodes and the spacer elements areprovided at regularly spaced contact points.

According to one embodiment, at least some of the spacer elementscomprise: a highly-reflective specular-reflecting adhesion layer of Alor Ag, a first protecting layer of a high-melting metal like Crdeposited on said adhesion layer; a conducting metal layer (e.g. Al)deposited on said protecting layer; and a second protecting layer of analkaline-resistant metal like Cr, deposited on said conducting metallayer. Preferably, the Al or Ag adhesion layer associated with thespacer elements on the light guide plate and the row-directed spacerelements on the passive plate are partly deposited on the ITO layersassociated with the column and row electrodes, respectively, to formlocal electrical contact points, and are partly deposited aside from theITO layers directly on the glass substrate plates. The Al or Ag adhesionlayer associated with spacer elements that are to form part of the foilelectrode circuitry is deposited entirely and directly on the glass ofthe substrate plates. The presence of highly reflective Al or Ag in theadhesion layer allows only a minimized amount of light absorption tooccur with respect to light propagating through the light guide plate.The presence of a high-melting metal like Cr in the protecting layerprevents the occurrence of spacer element deformations that mayotherwise occur due to a thermal expansion mismatch between thesubstrate material (glass) and the metal layer adhering to the substratewhen the spacer elements are subjected to high processing temperatures,for example during CVD deposition (Chemical Vapor Deposition) or sputterdeposition of an inorganic insulating layer on the spacer elements.Spacer element deformations may express themselves as pronounced localmetallic protrusions (hillocks) from the top surfaces of the spacerelements facing the foil. Hillocks, denoting large spikes of aluminumthat can form when the aluminum is subjected to processing temperaturesabove 250° C., are due to differences in the thermal coefficients ofaluminum and the substrate glass. Such processing temperatures occur forexample during the deposition of an insulating SiO₂ layer by a CVDdeposition process on top of the electrodes. Hillock spikes can createunwanted electrical short-circuits.

The conducting metal layer deposited on said protecting layer serves tolower the overall electrical resistance of the spacer elements and maycomprise any metallic low-resistance material. Said second protectinglayer of an alkaline-resistant metal like Cr, deposited on saidconducting metal layer, serves to protect the spacer elements againstchemical attack from the alkaline photoresist-stripping liquids that arenormally used for stripping of photo-resist material from the spacerelements after having completed photolithographic structuring of thespacer elements on the plates.

Such a composite spacer element thus provides for excellent electricalconductivity, improved adhesion to the substrate as compared tohomogeneous aluminum spacer elements, and reduced chance ofhillocks-induced short-circuit formation between the foil electrode andthe row and column electrodes. By disposing the spacer elements onlypartly on the ITO of their associated column and row electrodes,respectively, it is possible to limit the overall darkened interfacearea between the spacer elements and the ITO and thus to reduce theextent of light loss and/or color shift affecting the light propagatingthrough the light guide plate.

The adhesion layer is preferably between 50 nm and 100 nm thick, inorder to make the layer sufficiently thick for maintaining a highreflectivity, thus avoiding light-absorption induced light losses in thelight guide plate, while keeping it sufficiently thin to avoid spacerdeformations (hillocks) caused by the exposure of the spacer elements toelevated processing temperatures.

The first protecting layer is preferably at least 100 nm thick in orderto minimize the chance of spacer deformations (i.e. hillocks formations)when the spacer elements are subjected to elevated processingtemperatures during the physical deposition of an insulating layer onthe spacer elements and their associated ITO electrodes.

The conducting metal layer is preferably between 0.5 μm and 1.5 μmthick, the lower limit being set to maximize the spacer elementconductivity, the upper limit being set equal to the maximum allowedspacer height.

According to some embodiments, the conducting metal layer is constitutedby the first protecting layer.

The second protecting layer is preferably at least 50 nm thick, thisminimum thickness being set to provide sufficient protection to thespacer elements against chemical attack by alkalinephotoresist-stripping fluids during the wet-chemicalprocessing/structuring of the spacer elements. According to someembodiments, the second protection layer is constituted by theconducting metal layer and possibly also the first protecting layer toform one single-material layer.

According to a preferred embodiment, an additional electrical insulatinglayer is deposited on top of the spacer elements on the light guideplate and on top of the row-directed spacer elements on the passiveplate, so as to improve the electrical insulation of the foil electrodefrom said spacer elements on the active plate and from said row-directedspacer elements on the passive plate. Preferably, the additionalelectrical insulating layer is an inorganic layer measuring at least 100nm in thickness. Thereby short circuits between the foil electrode andthe spacer elements on the light guide plate and/or the row-directedspacer elements on the passive plate are effectively eliminated.

In another embodiment, the additional insulating layer is a continuouslayer deposited on top of every spacer element on the light guide plate,on top of every row-directed spacer element on the passive plate, and ontop of every transparent electrode on said light guide plate and saidpassive plate. In yet another embodiment, the additional insulatinglayer is deposited only on the top surface (facing the foil) of everyspacer element on the light guide plate but is not deposited on the sidesurfaces of the spacer elements on the light guide plate that connectthe top surfaces of the spacer elements with the top surfaces of thetransparent electrodes on the light guide plate. The absence of theinsulating layer at the side surfaces of the spacer elements on thelight guide plate prevents unwanted light leakage from the interior ofthe light guide plate via the insulating layer towards the outside.

Combining the spacer elements on the light guide plate and therow-directed spacer elements on the passive plate with the transparentcolumn and row electrodes, respectively, thus provides a number ofadvantages, including:

The aperture of each pixel can be increased, since the additional priorart resistance-reducing tracks are eliminated from the ITO electrodes.

The height of the resistance-reducing track formed by the spacer is lesscritical; it can now be made as high as the height of the prior artspacers.

The chemical reaction between the aluminum adhesion layer and the ITO ofthe column and row electrodes is now confined to very limited areas,namely the surface areas of the contact points needed in order toprovide proper electrical contact between the spacer and the associatedITO layer. In total, the total light absorbing aluminum/ITO contact areaon the light guide is now substantially reduced.

The adhesion problem between the resistance-reducing tracks and the ITOlayers are eliminated, since the adhesion now occurs mainly on the glassof the light guide plate and on the glass of the passive plate.

The total number of process steps is reduced, since spacers andresistance-reducing tracks can be provided for simultaneously in onesingle process.

The combination of the column-directed spacer elements on the passiveplate with the transparent foil electrode has the advantage of reducingthe electrical resistance of the foil electrode without having toprovide for resistance-reducing tracks directly on the foil itself.

In the following various embodiments of the invention will be furtherdescribed with reference to the accompanying drawings, on which:

FIG. 1 shows a cross-section of a prior art dynamic foil display, and amagnification of a single pixel element.

FIG. 2 shows a top view of an inventive column electrode circuitry.

FIGS. 3 and 4 show cross sections of the column electrode circuitry ofFIG. 2.

FIG. 5 shows a top view of an inventive row electrode circuitry.

FIGS. 6 and 7 show cross sections of the row electrode circuitry of FIG.5.

FIG. 8 shows embodiments of an inventive composite spacer element.

FIG. 9 shows a cross-section of an inventive display pixel element.

In FIG. 1 a cross section of a prior art foil display 100 is shown. Thedisplay comprises a light guide 101 and a passive plate 102. Betweenthese substrates a light scattering foil 103 is arranged, which isseparated from the substrates by means of spacers 104. A columnelectrode circuitry 105 formed out of ITO is arranged on the light guide101, and a row electrode circuitry 106 is formed on the passive plate102. A light source 107 connected to the light guide 101 isschematically shown, as well as light rays 108 traveling in the lightguide 101. Also shown is a cross section of an enlarged portion of thedisplay 100, illustrating a single activated pixel in further detail.The enlargement further shows a transparent foil electrode 109,deposited on the passive plate side of the foil 103. The enlarged pixelshown is in an activated state, the foil thus being in contact with thelight guide and consequently decoupling light from the light guide.

In FIG. 2 a top view of an inventive column circuitry on the light guideplate is shown. The shaded areas 201 indicate layers of ITO and thestripes 202 indicate conductive spacer elements embodied as compositetracks comprising an aluminum adhesion layer. The spacer elements aredirected parallel to the ITO electrode layers, but branches 205 offacross a neighboring ITO layer with regular intervals, each intervaldelimiting a pixel. In order to reduce the contact area between thespacer elements 202, 205 and the ITO layers 201, and thus the Al/ITOinterface area forming darkened light-absorbing regions, openings 203are provided in the ITO layers. The spacer elements thus only contactingthe ITO layers at the outer edge portions of the ITO electrode layers.As a result, the direct ohmic contact between the aluminum and the ITOis limited to only small surface areas and the effect of the chemicalreaction between ITO and aluminum and thus the loss and/or discolorationof light in the light guide is kept to a minimum. Cross section A′-A′illustrates ITO layers 201 and spacer elements 202, deposited on a glasslight guide 204. Cross section A″-A″ illustrates conductive spacerelement branches 205 partially covering the ITO layer, thus providingelectrical contact, and partially being deposited in the openings 203.Cross section B-B illustrates, from the transversal direction, thespacer element track 202 being deposited in the openings 203.

In FIG. 3 a schematic cross section of the spacer branch 205 of FIG. 2is shown. The spacer comprises an electrically conducting spacer element205 arranged on the electrode 201 which in turn is arranged on a lightguide plate 301. On the top surface of the spacer element 205 as well ason the electrode 201, electrically insulating layers 304, 305 aredeposited in order to provide robust insulation from the foil 306 whichis to be arranged on top of the spacer. In FIG. 4 a schematic crosssection of the spacer element 202 of FIG. 2 is shown. The spacer elementis arranged directly on the light guide plate 301 and is covered by aninsulating layer 304. No insulation is needed on the light guide plateitself. However, for ease of manufacturing the insulating layer 305arranged on the electrode 201 might also extend across regions of thelight guide which is not covered by electrode material.

In FIG. 5 a top view of two inventive row electrodes for a passive plateare shown. The six ITO areas 501 each define a separate pixel. Crosssection A-A shows the conductive row-directed spacer elements 502 thatare locally in electrical contact with the transparent ITO layers 501associated with the row electrodes. As can be seen, the ITO electrodesare structured so as to restrict the contact area between ITO and thespacers. Thus, also here the direct contact interface between the ITOand the aluminum of the row-directed spacer element is kept to a minimumin order to maintain a sufficiently strong adhesion between therow-directed spacer elements and the passive plate (but is of coursechosen to be sufficiently large so that proper ohmic contact isestablished). The column-directed spacer elements 503 plate are locatedin between adjacent ITO electrode layers 501 and are electricallyinsulated from both the row-directed spacer elements 502 and the ITOelectrode layers 501.

In FIG. 6 a schematic cross section of the spacer element 502 of FIG. 5is shown. The spacer element is partially deposited on the row electrode501 and partially deposited directly on the passive plate 601. Alsoshown are a foil 603 and a foil electrode 604, which are subsequently tobe deposited on the spacer element. In order to provide for robustinsulation of the row electrode from the foil electrode, an insulatinglayer 602 is encapsulating the spacer 502 as well as the electrode 501.Alternatively, the insulation on the side portions of the spacer can beomitted. In FIG. 7 is shown a schematic cross section of the columndirected spacer 503 of FIG. 5. The spacer is arranged on the passiveplate 601, and the foil 603 and foil electrode 604 are subsequentlyarranged directly on the spacer 503, in order to establish electricalcontact between the spacer 503 and the foil electrode 604.

The spacer elements can be formed out of a homogenous, electricallyconducting material such as aluminum. However, as stated above suchspacer elements are related to a number of problems. Therefore, acomposite resistance-reducing spacer element as shown in FIG. 8 isadvantageous for many applications. The composite spacer is arranged ona glass substrate 801 and comprises an adhesion layer of aluminum 802,which is partly in adhesive contact with underlying ITO tracks 806 andpartly in direct contact with the glass substrate 801. A firstprotecting layer 803 of chromium covers the aluminum adhesion layer 802.On top of the first protecting chromium layer 803 a substantiallythicker conducting metal layer 804 is deposited, for example comprisingaluminum or chromium, which in turn is covered by a second protectingalkaline-resistant chromium layer 805. Such a composite spacer elementcan be used on either or both of the light guide and passive plates. Inorder to reduce the light absorption from the light guide, it is highlyadvantageous to use composite spacer elements at least for the lightguide spacers.

A spacer as shown in FIG. 8 can be manufactured as follows: First a thinITO column electrode layer (30 nm or less) is structured on the lightguide. Then spacer elements on the light guide plate are structuredconsisting of: A thin adhesion layer of aluminum (50-100 nm thick, alsoAg, Mg or a combination of these elements can be used such as to providea high reflectivity) partially in contact with the ITO column electrodelayer, a thin protecting layer of Cr (100-200 nm thick, any high meltingpoint metal will do) to suppress the formation of hillocks from theunderlying aluminum adhesion layer that may arise when the spacerelements are exposed to a high processing temperature, a thickconducting aluminum layer (0.5-1.5 μm thick, or any other metal will do(e.g. Cu or Cr)) to form the spacer elements and to create the desiredlow resistivity of the spacer elements, and finally a thinalkaline-resistant Cr-layer (50-100 nm thick) to provide protection ofthe spacer element against chemical attack by alkaline resist-strippingfluids during the wet-chemical processing/structuring of the spacerelements.

It is not possible to deposit 1 μm thick aluminum spacer elementsdirectly on ITO due to adherence problems and the traditional adherencelayers (like Cr and Ti) cannot be used due to their high lightabsorption, which would lead to a substantial loss of light intensityinside the light guide.

Spacer elements on the passive plate can be manufactured in the same wayas the spacer elements on the light guide plate but the row-directedspacers are brought into contact with the ITO of the row-electrodelayers while the column-directed spacer elements on the passive plateare deposited directly on the glass of the passive plate and are therebykept electrically insulated from both the ITO row electrode layers andthe row-directed spacer elements on the passive plate.

A dielectric insulating layer is preferably deposited on therow-directed spacer elements and on the ITO layers associated with therow electrodes on the passive plate, in order to electrically insulatethe row electrodes from the foil electrode. Preferably also the ITOlayers associated with the column electrodes and the top surfaces of thespacer elements on the light guide plate are covered with a dielectricinsulating layer, in order to electrically insulate the columnelectrodes from the foil electrode. The dielectric insulating layerpreferably consists of an inorganic oxide or an inorganic nitridematerial e.g. Al₂O₃, Si₃N₄, TiO₂ or SiO₂.

A side view of a single foil display pixel 900 in the OFF-state is shownin FIG. 9. The pixel is arranged on a light guide 905 on which columnspacers 904 and a column electrode 911 are arranged. Insulating layers912 and 913 (e.g. SiO₂) are arranged on the column electrode 912 and thecolumn spacers 904, respectively, and separate the column circuitry fromthe foil 903 and foil electrode 907. Alternatively, the insulating layercan be omitted and the foil itself thus being the insulating mediumbetween the column electrode and the foil electrode. In order to preventlight leakages from the light guide, the insulating layer is restrictedto the lateral surfaces of the column circuitry, i.e. the top surfacesof the spacer elements and the top surfaces of the ITO layers, leavingthe side surfaces of the spacer elements open. Otherwise a certainamount of light will be decoupled from the light guide via theinsulating layer even if the pixel is in its OFF-state. Passive platecolumn spacers 902 and a row electrode 901 are arranged on the passiveplate 910, separated from each other. An insulating layer 908 covers therow electrode 901 whereas the passive plate column spacers 902 are indirect electrical contact with the foil electrode 907.

The present invention thus reduces the number of processing steps,increases the pixel aperture because no space is occupied by separateresistance reducing tracks, and minimizes the row and column electrodeRC time because the resistance reduction by the spacers is much largerthan what can be accomplished with the previously discussed separateresistance-reducing tracks on ITO.

It is to be understood that the skilled man readily envisages manyvariations of the present invention. For example, the row and columnconfiguration can of course be inverted, the row circuitry thus beingarranged on the light guide and the column circuitry being arranged onthe passive plate. It is of course also possible to invert the foil andfoil electrode, the foil electrode thus being deposited on the lightguide side of the foil, alternatively, the foil can also behomogeneously made out of a conducting material or insulated on bothsides from the row and column circuitry by a separate insulating layer.

In summary, a dynamic foil display 900 is provided wherein the spacerelements 902, 904 also serve as resistance-reducing tracks for theelectrode circuitry. Resistance-reduction can thereby be provided forthe row and column electrodes as well as for the foil electrode withoutarranging separate resistance-reducing tracks. Thereby the addressingperformance of the display is substantially improved and themanufacturing of the display simplified. To this end a foil display 900comprises a light guide plate 905; a passive plate 910; and atransparent light-scattering foil 903 sandwiched between and separatedfrom said plates by means of spacer elements 902, 904 is described. Thespacer elements 902, 904 are arranged essentially along rows and columnson said plates 905, 910 and thereby define a plurality of pixel elementsarranged in a matrix configuration along said rows and columns. Thedisplay further comprises an electrode circuitry which comprises atransparent foil electrode 907 arranged laterally on said foil andtransparent row and column electrodes 901, 911 arranged along said rowsand columns on said light guide and passive plates; and the spacerelements 902, 904 are electrically conducting and interconnected withsaid electrodes 907, 911, 901 and thus form part of said electrodecircuitry.

1. A foil display (900) comprising a light guide plate (905); a passiveplate (910); a transparent light-scattering foil (903) sandwichedbetween and separated from said plates by means of spacer elements (920,904), said spacer elements being arranged essentially along rows andcolumns on said plates and thereby defining a plurality of pixelelements arranged in a matrix configuration along said rows and columns;an electrode circuitry comprising a transparent foil electrode (907)arranged laterally on said foil and transparent row and columnelectrodes (901, 911) arranged along said rows and columns on said lightguide and passive plates; characterized in that said spacer elements areelectrically conducting and interconnected with said electrodes and thusform part of said electrode circuitry.
 2. A foil display according toclaim 1, wherein some of the spacer elements (502) are essentiallyparallel with said row electrodes (501) and electrically interconnectedtherewith.
 3. A foil display according to claim 1, wherein some of thespacer elements (202) are essentially parallel with said columnelectrodes (201) and electrically interconnected therewith.
 4. A foildisplay according to claim 1, wherein some of the spacer elements (503)on at least one of said plates are electrically interconnected with saidfoil electrode and are spatially separated from and extend in adirection essentially transversal to the remaining spacer elements onsaid at least one plate.
 5. A foil display according to claim 1, whereinsaid transparent row and column electrodes are formed out of ITO (901,911).
 6. A foil display according to claim 1, wherein said spacerelements (902, 904) comprise aluminum.
 7. A foil display according toclaim 2 or 3, wherein said spacer elements (202, 502) electricallyinterconnected with said row or column electrodes (201, 501) aredeposited partially on the respective transparent electrodes andpartially on the respective plate.
 8. A foil display according to claim1, wherein the spacer elements comprise: a reflective adhesion layer(802) of Al or Ag, arranged at least partially on a transparentelectrode layer (806); a first protecting layer (803) of a high-meltingpoint metal, deposited on said adhesion layer; a conducting metal layer(804), deposited on said first protecting layer; and a second protectinglayer (805) of an alkaline-resistant metal, deposited on said conductingmetal layer.
 9. A foil display according to claim 8, wherein saidadhesion layer (802) is between 50 nm and 100 nm thick.
 10. A foildisplay according to claim 8, wherein said first protecting layer (803)is at least 100 nm thick.
 11. A foil display according to claim 8,wherein said first protecting layer (803) is formed out of Cr.
 12. Afoil display according to claim 8, wherein said conducting metal layer(804) is between 0.5 μm and 1.5 μm thick.
 13. A foil display accordingto claim 8, wherein said conducting metal layer (804) is formed out ofAl.
 14. A foil display according to claim 8, wherein said secondprotecting layer (805) is at least 50 nm thick.
 15. A foil displayaccording to claim 8, wherein said second protecting layer (805) isformed out of Cr.
 16. A foil display according to claim 1, wherein aninsulating layer is deposited on said foil electrode, so as to insulatesaid foil electrode from said spacer elements and said transparentelectrodes.
 17. A foil display according to claim 1, wherein insulatinglayers (304; 602) are deposited on top of at least parts of said spacerelements.
 18. A foil display according to claim 17, wherein a continuousinsulating layer (602) is deposited on top of every spacer element andevery transparent electrode on said passive plate.